1. Field of the Invention
A method and related communication device used in a wireless communication system is disclosed and more particularly, to a method of setting radio access technology (RAT) capability in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of user equipments (UEs) each including a mobile entity and a USIM (Universal Subscriber Module). A core network, i.e. Evolved Packet Core (EPC), is responsible for UE registration, authentication, handover mobility, etc, through the E-UTRAN.
A UE with multi-modes supports multiple radio access technologies (RATs), such as GSM (Global System for Mobile communications), UMTS (Universal Mobile Telecommunications System), and LTE systems. A connection in LTE can be established by the eNB only if the UE has a normal service capability in the LTE cell. The normal service capability herein means that the UE has a USIM inserted and the core network accepts the registration of the UE. In other words, the inter-RAT handover is inapplicable for the UE with an invalid USIM.
On the other hand, the multi-mode UE tends to always select a LTE cell instead of a cell of other RATs (a non-LTE cell) due to system/procedure design (e.g. a measurement report). One of designs is that a measurement report of the UE is generated based on a measuring algorithm that always results in higher performance result in the LTE cell measurement over other RAT cell measurement. Such measurement result always leads the UE to select/reselect a LTE cell. As a result, it is difficult for the UE with the invalid USIM to select the non-LTE cell during cell selection. This affects emergency service provision.
Take an emergency call service for example. The emergency call service is not supported in the LTE system, but is supported in the GSM and UMTS systems. When the UE camping on a LTE cell sends an emergency call request, an eNB of the LTE cell requests the UE to perform a handover to camp on a GERAN (GSM/EDGE Radio Access Network) cell or a UTRAN (UMTS Terrestrial Radio Access Network) cell. The eNB also transfers the handover request to the core network that responds with a reject message indicating that the UE does not have the normal service capability. In this situation, the eNB cannot initiate the handover for the UE due to the reject message. The UE has no way to access to the GERAN or UTRAN after receiving the reject message from the network. The procedure to establish the emergency call is therefore abandoned.
The emergency call service demands urgency of a fast-established connection and is specified by national requirements, e.g. FCC (Federal Communications Commission). In the prior art, the multimode UE without normal service fails in emergency call establishment in the LTE system.